[Meiko]

Among "older" actors and bigger names:

John Malkovich - He can play the psycho! He has that kind of pure evil expression, plus he's a good actor, I think he could pull it off.
Tim Roth - An actor who's very much the type of guy you can hate. In many movies he played the evil or twisted character. I can perfectly see him as the new Joker.
Willem Dafoe - This man is just plain creepy and looks pretty much as the sadistic villain that is the Joker. He has the physical attributes and the facial features for it.

Among the younger and less famous:

James Franco - In Spiderman till now he hasn't done much, but I feel if given the opportunity he could show the guts and the strenght for a real villain.
Daniel Craig - In Road to Perdition he was great as the vicious son, he's got the evil expression and twisted look.
Hayden Christensen - He demonstrated he can play the wicked guy in SW. He's tall and strong enough for action scenes, so he looks ok to me to confront Bale's Batman.